Parsamian 21 = HH 221 = IRAS 19266+0932
Young Stellar Object (YSO) in Aquila
RA: 19:29:01
Dec: +09:38:43
Size: 1' x 0.3'
Mag (V): 14.07

Who is Parsamian? [1,2]
Prof. Elma Suren Parsamian is one of the foremost Armenian astronomers affiliated with the famous Byurakan Astrophysical Observatory in Armenia. She got her PhD under Victor Ambartsumian, and has worked at Byurakan, Yerevan State University (Armenia), Tonantzintla Observatory of Mexico, and at the Max Planck Institute for Astronomy at Heidelberg.

Parsamian's work centers around the physics of gas nebulae, investigation of variable stars in clusters and associations, and archeoastronomy. Notably, in 1965, she published her first catalog of cometary nebulae found on Palomar plates, the best-known object in this catalog being the subject of this post [3]. The catalog seems to have been later revised with Petrosyan [4, 5]. Many of the "PP" objects are more familiar through other designations on this forum, like Hubble's Variable Nebula, Hind's Variable Nebula, and NGC 1999. In addition, she worked on flare stars (variable stars that flare up really bright unpredictably), including with Guillermo Haro of Herbig-Haro fame. She is very well-known for identifying the astronomical significance of the "Armenian Stonehenge", Zorats Karer, which has since been challenged and remains controversial [6].

parsamian.jpg
Elma Parsamian, from a website

What is Parsamian 21? [7]
The current model of Parsamian 21 is as follows: HBC 687, an eruptive young star of "FU Orionis" type (a "FUor" for short) lies at its heart . These pre—main sequence stars can brighten up by 4—5 magnitudes in a matter of months and remain bright for several years or decades.

FUOriLightCurve.jpg
FU Orionis Light Curve
Credit: User PopePompus from Wikimedia

These stars are thought to comprise of an accretion disk around a young, low-mass (<~ 2 Msun) star (a "T Tauri" star). Once the protostar becomes hot enough to ionize hydrogen, the rate of accretion can drastically increase due to an instability, which leads to a sudden inflow of lots of mass onto the star, leading to a hot and bright inner accretion disk. It is this hot accretion disk that is thought responsible for the sudden brightening [7,8]. Eventually, the material in the inner disk gets depleted into the star and the system becomes dim again until new material accumulates. These outbursts are thought to recur over thousands of years, although of course we have no human records of observing such recurrences. Even though no outburst of HBC 687 has been observed, the indirect evidence suggests that it is a FUor.

Parsamian_21_POSSIIBlue.png
POSSII Blue image of Parsamian 21. Credit: DSS/MAST/STScI

Young T Tauri stars with accretion disks are known to eject material along the axis perpendicular to the disk, in a bipolar outflow. Herbig-Haro objects are formed when the bipolar outflow collides with the surrounding interstellar medium (See the OOTWs on HH1, HH2 and HH 222). The H-alpha image presented in [7,9] shows the Herbig-Haro object embedded in the system, extending north and south of the star:

Parsamian_21_H_Alpha.png
Halpha image of Parsmian 21 from [7,9]

Coming back to the cometary nebula, it is bipolar although the southern part is very very weak, with the southeastern jet more prominent than the southwestern jet. The southeastern jet is seen in the near-infrared HST image below, and also vaguely in the above POSSII image. The prominent northern nebulosity is loop-shaped as is seen in both images.

Parsamian_21.jpg
Near-infrared image of Parsamian 21 from the HST

Here's the hypothesis for what causes its structure, explained through a diagram in [7,9]:

Parsamian21_Hypothesis.jpeg
Sketch from [7,9] showing how the morphology of Parsamian 21 may have been formed

The bipolar outflow excavates a cavity in the dense material surrounding the star, which is then illuminated by the star. We see the light scattered from the walls of the cavity as the cometary nebula.

Visual Observation

I came across this amazing object on Larry Mitchell's Texas Star Party Advanced Observing List from 2014. I was at TSP that year with my 18", and was working through Larry's list. Anyone who has had that experience with an 18" knows that typically, Larry's objects can sometimes be exceedingly tough for such apertures. Notably that year, the Serpens Red Object was on the list. So on May 29th 2014, when I landed on Parsamian 21 after logging the Serpens Red Object, I was pleasantly surprised at how bright and actually visible it was! I logged it as "Beautiful! Quite bright. Elongation easily detected". I somehow have this view of Parsamian 21 as a pencil of light shooting out of a bright point burned in my memory, although after 8 years, who knows how accurate it is — it's time for a refresher.

The challenge for large telescopes is to view the loop that was mentioned earlier. If I understood him correctly, Jimi says that he has seen this feature with his 48".

One of the joys of writing (and hopefully reading) these OOTW posts is that it makes you learn so much more about an object, that you can go back to your observations and breathe so much more meaning into those 3 pops of a faint fuzzy blob. But what's even better is when both the visual view and the astrophysics are amazing, as is with this object, so why not

GIVE IT A GO, AND LET US KNOW!

References:
[1] Biography of Elma Parsamian: https://www.aras.am/Books/books/Elma-Parsamian-Eng.pdf
[2] https://www.aras.am/FamousAstronomers/parsamian.html
[3] Parsamian's original catalog: only abstract is available in English https://ui.adsabs.harvard.edu/abs/19....146P/abstract
[4] Parsamian-Petrosyan catalog: https://arar.sci.am/dlibra/publicati...110062/content
[5] Presumably an English translation of [4] with contemporary digitized imagery: https://arar.sci.am/dlibra/publicati...295324/content
[6] Armenian stonehenge: https://www.smithsonianmag.com/trave...nge-180964207/
[7] Agnes Kospal's thesis: http://kisag.konkoly.hu/Group/events..._theses_en.pdf
[8] https://www.annualreviews.org/doi/pd...astro.34.1.207
[9] https://academic.oup.com/mnras/artic...3/1015/1035688